Studies On Effects Of Nitrogen And Phosphorus On Phytoplankton From The Surface Microlayer(SML)

The sea surface microlayer(SML) is a thin layer between sea surface and the air, which has unique physicochemical and biological properties. Water samples from the SML were collected from Daya Bay, the South China Sea, between June and December 2013 to understand the phytoplankton community structure by pigment analysis. The cholorophyll a contents, nutrient utilization, and pigment composition were examined after addition of nutrients to the SML samples. Water samples were collected from the SML from Daya Bay in May and July 2014, Guishan Island sea area in October 2014 and Guanghai Bay in December 2014, and to determine alkaline phosphatase activity(APA) and its relationships with nutrients. Further, the responses of APA to organic and inorganic P were studied. The purpose of this study is to understand the potential changes of phytoplankton community in the SML under eutrophication. The results of this study provide basic data for the protection of costal environments and ecosystems.Results showed that zeaxanthin was the dominant pigment of the SML in Daya Bay, and next was fucoxanthin. The result indicated that cyanobacteria and diatoms dominated the phytoplankton community in the SML. The proportions of cyanobacteria were higher in high temperature seasons such as summer and autumn, while diatoms dominated in winter. However, phytoplankton community in the SML from the Guishan Island sea area and Guanghai Bay was dominated by diatoms and then cyanobacteria. The study suggested that phytoplankton community structure of the SML might have the potential to change from diatoms-dominated to cyanobacteria- dominated.The biomass of phytoplankton increased rapidly after addition of inorganic N and P. However, the growth could not be accelerated in cultures in which N or P was added alone. After addition of inorganic nutrients, the phytoplankton community structure changed from cyanobacteria-dominated to diatoms-dominated, reflected by the rapid increase in concentrations and proportions of fucoxanthin, which generally represents diatoms biomass, and the decrease of zeaxanthin, the pigments representing those of cyanobacteria. The proportions of zeaxanthin increased at the late period of the experiment as the consumption of nutrients. The phytoplankton in the SML could utilize all kinds of organic N and P, and diatoms dominated the phytoplankton community after nutrients addition, followed by dinoflagellates and haptophytes. Furthermore, cyanobacteria and dinoflagellates increased significantly at the late period of the experiment. Results suggested that diatoms were suitable to grow in eutrophic conditions; however the risks for harmful algal blooms, e.g. those of cyanobacteria, increased after diatom blooms as the depletion of nutrients particularly for Si depletion.The phytoplankton community was limited by phosphorus in Daya Bay, Guishan Island sea area and Guanghai Bay in the surveys of APA. The ranges of total APA were from 1.41 to 35.26 nmol/L?h. The free APA ranged from 0 to 14.99 nmol/L?h, bacterial APA from 0.03 to 22.9 nmol/L?h, and phytoplankton APA from 0.19 to 24.01 nmol/L?h. Results of alkaline phosphatase(AP) analysis showed that phytoplankton was the main contributor of AP.Diatoms could become the dominant group shortly after the addition of inorganic or organic P, and dioflagellates occurred in low numbers at the late period of the experiments. The total APA and APA of all particle size in the cultures after nutrient addition were higher than those of natural sea waters. Furthermore all APA showed significant negative correlations with DIP and DTP concentrations, which suggested that P limitation could promote AP activity. As phytoplankton APA was the major contributor to the total APA and diatoms dominated in the phytoplankton community, diatoms should have strong AP activity.